V,
Try looking at this.....
https://www.google.com/search?q=powerstat+bp+2116&biw=1600&bih=783&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjQ3dTz3vbPAhUh7IMKHQZ9CZkQ_AUICSgE&dpr=1#imgrc=sfVpU8EjFx8PmM%3A Forget about your particular devise for the moment. Look at the diagram in the above link. Note that the "common" terminal appears in both the input and output.
The winding of an auto transformer is common to the input and the output. Kinda like a taped loading coil on a vertical antenner. Anyway, you can control the voltage up to the 'line' voltage with one connection, or over the line voltage with the other. You have (2) phases ( I hate the way 'they' use " phase " ) which gives you 240 V. Watt I do, for a few things, is to use a 6.3 volt transformer in place of 120V or 240V AC. I'll use the clip lead method and figure things out. ( Also, I use lots of Scotch SUPER 88 on the leads.)
I used these things in 'Lectrical' class, but its been over ~40 years so I'm a bit foggy. Not a clean answer, but you can probably figure things out.
KLC
